This study was conducted to examine in vitro antigenotoxic effects of the peptides derived from the hydrolysates of silk fibroin in mouse embryo 3T3 cells. The hydrolysates were prepared by acid or enzymic hydrolysis of fibroin, and antigenotoxicities were determined by measuring the reduced levels of DNA damage using the Comet assay. The fibroin preparation isolated from cocoons was efficiently digested by hydrolytic reactions with acid or Alcalase, an industrial protease. The acid- and Alcalase hydrolysates showed higher antigenotoxic activities than peptic- and tryptic hydrolysates. Active peptide fractions were separated from acid- and Alcalase hydrolysate by gel filtration chromatography. It was deduced from the chromatograms and amino acid analyses that the size (3-7 amino acids) and the glycine plus alanine content of peptides might be important factors for their activities. In addition, it was found that the antigenotoxic effect of peptides was due both to the protective interactions between cells and peptide molecules and to the direct inactivation of the mutagen, MNNG by peptides. Considering the availability and safety of silk fibroin and the superior antigenotoxic effects of produced peptides, the results of this study showed the possibility of utilizing fibroin as a source for chemopreventive functional peptides. (C) 2002 Elsevier Science Ltd. All rights reserved.